1. Field of Use
This invention relates to a process for recovering TiO.sub.2 from ilmenite-type ores by digestion with hydrofluoric acid, in which hydrofluoric acid and ammonium hydroxide are recovered by an electrodialytic water-splitting process from by-product aqueous ammonium fluoride, and recycled.
2. Brief Description of the Prior Art
Processes for recovering TiO.sub.2 from ilmenite-type ores by digesting the ore with hydrofluoric acid or ammonium fluoride are well known in the art.
U.S. Pat. No. 1,501,587 (1924) describes the use of aqueous hydrofluoric acid for digesting ilmenite-type ores to recover pigmentary titanium dioxide in which the hydrofluoric acid may be recovered by acidification of aqueous by-product fluoride solutions followed by distillation.
U.S. Pat. No. 2,042,435 (1936) describes the use of ammonium fluoride as an aqueous solution or as a dry reagent for the digestion of ilmenite-type ores. Following digestion with either reagent, excess ammonium fluoride is volatilized off and the titanium values in the aqueous solution are leached out with water or a non-alkaline solution of aqueous ammonium fluoride. Iron impurities are filtered off, the solution is neutralized with ammonia and then treated with a soluble sulfide to precipitate the remainder of the iron impurities and other heavy metal impurities, which were not filtered off initially. Aqueous ammonia is then added to precipitate hydrated titanium dioxide which is then filtered and calcined to produce pigmentary TiO.sub.2. The by-product solutions of ammonium fluoride and ammonium hydroxide can be combined and then recycled for use in the digestion step. However, the solutions must be evaporated to obtain ammonium fluoride as a dry reagent for use in the preferred process.
U.S. Pat. No. 3,787,304 (1974) describes a process for converting waste fluosilicic acid into hydrogen fluoride by treating an aqueous solution of fluosilicic acid with an alkali hydroxide to form a solution of alkali fluorides. The solution of alkali fluorides is subjected to the process of electrodialytic water-splitting to form an aqueous solution of alkali hydroxide and an aqueous solution of hydrogen fluoride from which anhydrous hydrogen fluoride may be recovered.
What is needed and what the prior art does not provide is a process for recovering pigmentary TiO.sub.2 from ilmenite-type ores by digestion with hydrofluoric acid, wherein recovery and recycle of all necessary liquid reagents can be performed without a separate distillation step and where the process contains no significant effluent streams.